Just an update...

So I got the YouTube Enhancer - Tweak Tube. It's now in Extensions in Addons in my Firefox settings top left of screen.

Maybe it's just me, but it was all pretty complicated. Trial and error really. There are dozens of settings in the extension so I had to try all of them to see what would change when I refreshed YouTube each time.

I've finally got it working so that all I get on the computer screen is a much larger than usual YouTube screen showing the song or the movie clip. No comments, no ads (yet), no lists and previews of other songs although that may be just the way I have the settings.

I'll keep playing with it when I get time.